Add touchscreen SYN_DROPPED test

The test bombards the evdev kernel driver to generate SYN_DROPPED event(s)
as the queue inside will be filled up quickly. Therefore, we should be able
to see the SYN_DROPPED event and process it inside xf86-input-evdev driver
by examining the log file Xorg.0.log.

BUG=chromium-os:35291
TEST=on device; follow the steps
  1. Modify the line in /sbin/xstart.sh to change the nice level from -20 to 19.

     exec nice -n 19 /usr/bin/X -noreset -nolisten tcp vt01 -auth $1 2> /dev/null

  2. Restart the X server to slow it down.
     restart ui

  3. Execute the test script to genereate the SYN_DROPPED event and see if it passes the test.
     cd /usr/local/autotest
     bin/autotest tests/platform_TouchscreenSynDrop/control

  4. Test if touchscreen still functions correctly.

Change-Id: I4e2d899ef5d5af5ad4cb8835b76c7f3d48af7715
Reviewed-on: https://gerrit.chromium.org/gerrit/36896
Reviewed-by: Joseph Shyh-In Hwang <josephsih@chromium.org>
Commit-Ready: Chung-yih Wang <cywang@chromium.org>
Reviewed-by: Chung-yih Wang <cywang@chromium.org>
Tested-by: Chung-yih Wang <cywang@chromium.org>
3 files changed